1. Field of the Invention
The present invention relates to a method for producing ultrafine powder, and more particularly, to a method for producing nano-sized titanium dioxide (TiO2) ultrafine powder from titanium tetrachloride (TiCl4) in the vapor phase by the gas phase oxidation reaction using flames.
2. Description of the Related Art
Nano-sized ultrafine powder, which refers to a powder having a particle size of less than 50 nm, is widely used as a new material due to its large specific surface area per unit weight and high activity.
For example, nano-sized titanium dioxide ultrafine powder is used as a high-quality pigment and a photocatalyst, as well as cosmetics, medicines and coating material for transparent sound proof plates.
There are two conventional methods to produce nano-sized titanium dioxide (hereinafter, referred to as xe2x80x9cTiO2xe2x80x9d) ultrafine powder: a physical method that comprises vaporizing a metal by heating and condensing the metal vapor into ultrafine powder, and a chemical method that involves the chemical reaction of metal compounds.
The physical method for producing nano-sized TiO2 ultrafine powder requires great energy consumption for vaporization of the metal with the result of high production cost and low productivity but makes it possible to product high-purity powder. On the other hand, the chemical method provides low-purity powder at a low production cost with high productivity, and includes a gas phase method and a liquid phase method.
Now, a description will be given as to a method for producing nano-sized TiO2 ultrafine powder by the chemical method related to the present invention.
In the preparation of nano-sized TiO2 ultrafine powder by the gas phase chemical reaction method, it is necessary to provide a high temperature of at least 1000xc2x0 C. and large gas flow rate. For this purpose, an approach for providing reaction conditions in the gas phase and high temperature using flames is known, wherein flame temperature, gas flow rate, concentration of reactants, and additives are critical reaction parameters that control the size and crystal form of the primary particles in the preparation of ultrafine powder.
The conventional approach for producing ultrafine powder by the gas phase chemical reaction using flames is disclosed in U.S. Pat. No. 5,698,177 under the title of xe2x80x9cProcess for producing ceramic powders, especially titanium dioxide useful as a photocatalystxe2x80x9d as filed on Jun. 8, 1995; and U.S. Pat. No. 5,861,132 under the title of xe2x80x9cVapor phase flame process for making ceramic particles using a corona discharge electric fieldxe2x80x9d as filed on Sep. 4, 1997.
U.S. Pat. No. 5,698,177 suggests various methods in regard to the control of reaction parameters, the use of additives and the effect of the corona electric field formed over the burner of the reactor to produce TiO2 powder for photocatalyst in the reaction system composed of TiCl4, air, and hydrocarbon-based fuel gas. The claims of this patent define the preparation of TiO2 powder by the gas phase reaction of TiCl4 and oxygen, the flame reactor used in the preparation, how to inject the sample, the added amount of sample and air, the voltage of the electric field, and the amount of additives.
In addition, U.S. Pat. No. 5,861,132 discloses a process for producing powders of various metal oxides (e.g., silica, alumina, zirconia, etc.) including TiO2 powder with various flame reactors (e.g., pre-mixed flame reactor, turbulent flame reactor, or larminar diffusion flame reactor) while providing the corona electric field over the reactor.
In the preparation of nano-sized powder using the gas phase chemical reaction, the concentration of the sample in the reaction gas has to be considerably low and an excess of gas has to be introduced into the reaction region (flame) with increasing the added amount of the reactant in order to increase the yield of the nano-sized powder per unit time.
However, the reaction system of the combustion gas composed of TiCl4, air and hydrocarbon introduced into the three pipes as suggested in U.S. Pat. No. 5,698,177 has a problem that the linear velocity of the gas increases in the burner due to an excess of air to reduce the retention time of the reactant, thus resulting in existence of non-reacted materials and incomplete combustion of the fuel gas.
It is, therefore, an object of the present invention to solve the problem and to provide a method for producing nano-sized TiO2 ultrafine powder from TiCl4 using the reaction system of TiCl4-argon-hydrogen-oxygen-air, TiCl4-argon-hydrogen-air-air, or TiCl4-argon-hydrogen-oxygen/air-air with a five-piped turbulent diffusion flame reactor, the method comprising: maintaining a low concentration of the sample in the reaction gas, minimizing the amount of non-reacted materials and completely combusting the fuel gas (hydrogen) to increase the yield of the nano-sized powder per unit time.
To achieve the above object of the present invention, there is provided a method for producing nano-sized TiO2 ultrafine powder through oxidation with a mixed gas system passed through flames having a high temperature, the mixed gas system being composed of TiCl4-argon-hydrogen-oxygen-air, TiCl4-argon-hydrogen-air-air, or TiCl4-argon-hydrogen-oxygen/air-air and obtained from vaporization of a liquid reactant, TiCl4. In the preparation method, the TiO2 ultrafine powder can have optimum particle size and crystal form by varying critical parameters such as the TiCl4 content in the reaction gas, the flow rate and composition of the gas, and the like.